1. Field of the Invention
The present invention pertains to a method of producing a metal ball of Cu, etc., that is effective as a bump for semiconductor packages by heating and melting and then cooling a small metal piece, and a semiconductor package that uses the same ball. The present invention pertains to a method with which good relebiity from the jig during cooling after heating and melting and dimensional accuracy of the metal ball are guaranteed and productivity s markedly improved by, for instance, placing a metal piece on the jig with a layer of fine powder having low wettability with the metal piece in between.
2. Description of the Prior Art
Minute metal balls of Cu, Al, Ni, etc., are used as bumps for BGA (ball grid array)-type semiconductor packages, ornamental materials, wiring electrodes, relay contacts, etc. The method whereby a metal piece cut from a metal wire with dimensions similar to the intended diameter is heated and melted on a flat jig and then made into a sphere under surface tension (Japanese Patent Application No. 58-113586 (Japanese Patent Laid-Open No. 60-5804)) is a method of producing the same.
The method whereby metal pieces are individually shook into and arranged in a plurality of holes formed in a jig and these metal pieces are heated with the jig at a temperature that is the melting point or higher in a non-oxidizing atmosphere and melted is disclosed as a method of increasing sphericity further (WO 95/24113).
Moreover, the method whereby metal pieces are buried in powder that is mainly graphite powder and heated and melted to produce metal balls is presented as a simple method for efficient production (Japanese Patent Laid-Open No. 10-8113).
Methods that use the above-mentioned jig are excellent in terms of handling performance because large quantities of metal pieces can be handled at the same time. However, not only is dimensional accuracy inferior when the surface of the metal ball that comes into contact with the jig is flat, but releasability of the metal ball after melting also becomes poor with degradation of the jig, life of the jig is curtailed, and there is a reduction in production efficiency. This tendency is particularly obvious with a minute diameter of a diameter of 0.4 mm or less.
The surface state is poor because metal pieces are buried in powder by the above-mentioned production methods that use fine powder. Moreover, there are cases where dimensional accuracy is an error of .+-.0.1 m in terms of the desired dimensions.